The effect of excitotoxic hippocampal lesions on simple and conditional discrimination learning in the rat

The effect of excitotoxic lesions of the hippocampus on acquisition and rev ersal of simple and conditional tasks was investigated using a Y-maze. Hipp ocampal-lesioned rats were severely impaired on acquisition and reversal of a conditional visuo-spatial task (where different pairs of visually distin ctive choice arms indicated whether a left or right arm choice was correct on that trial) and were unable to acquire a visuo-visual conditional discri mination (where the appearance of the start arm indicated which of the visu ally distinctive choice arms was correct irrespective of their left/right p osition). They were not impaired on acquisition or reversal of a simple spa tial left/right discrimination task (where all arms had the same visual app earance) nor on acquisition of a visual discrimination (where the correct, visually distinctive, choice arm varied in its left/right position). Hippoc ampal-lesioned rats were, however, impaired on reversal of this visual disc rimination task and on acquisition and reversal of another visual discrimin ation task in which the visually distinctive choice arms were less differen t from each other than in the first version of this task. The degree of imp airment in the lesioned rats was related to task difficulty for the sham-op erated rats and was not specific to tasks requiring spatial choices, visual discrimination or conditional responding. The impairment on conditional ta sks was greater than the impairment on those non-conditional tasks which ha ppened to be matched for task difficulty for the sham-operated rats, sugges ting that the conditional demand may target the function of the hippocampus rather closely. Statistically worse than chance performance by hippocampal -lesioned (and sham-operated) rats at the beginning of reversal testing, wh ich was given 24 h after achieving criterion on acquisition of that task, i ndicated that hippocampal-lesioned rats simultaneously exhibited good memor y but impaired learning for the type of information required for those task s. (C) 1999 Elsevier Science B.V. All rights reserved.